Floating Covers to Reduce Gas Emissions from Liquid Manure Storages: A Review

Liquid manure (slurry) storages are sources of detrimental gases. Floating covers are a potential mitigation measure that can be implemented on many storage facility types. This article reviews the use of floating covers to reduce the emissions of odors, hydrogen sulfide (H2S), ammonia (NH3), and greenhouse gases (GHGs) including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Covers have been established with materials of natural origin (e.g. natural crusts, straw, peat, and light expanded clay aggregates), synthetic origin (e.g. geotextile, plastic, and rubber), and composites of both. Nearly all cover types have been capable of substantially reducing NH3 emissions (compared to uncovered controls). Reductions of odor and H2S have also been good, though fewer cover types have been assessed with respect to these parameters. When used alone, oil covers can produce foul odors and should not be used. Less information is available on the influence of covers on GHG emissions. In studies >2 weeks long, covers generally increased CH4 and CO2 emissions. All studies where N2O was measured found that permeable covers increased its emission. There is some difficulty comparing laboratory and field observations, which may be due to study duration, hydrologic influences, or slurry characteristics. Principles of mass transfer are discussed with respect to the mechanisms of cover operation. Though evidence of microbial gas consumption in permeable covers exists, its relative importance is unclear. Currently, information on many cover materials is limited to one or two studies, and simultaneous assessments of the effects on all aforementioned gases is lacking for all covers.